Cold rolledsteel sheet and method for manufacturing the same

ABSTRACT

A method of manufacturing a cold-rolled steel sheet includes: adding, by weight %, carbon (C) 0.005% or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1.0%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.30 or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%; adjusting the atomic ratio of Nb/C to 1 or more and the atomic ratio of Al/N to 0.5 to 1.5, homogenizing a steel containing iron (Fe) and elements inevitably contained in manufacturing the steel as the remainder at temperature of 1150 to 1300° C., setting the final hot-rolling temperature to 890 to 950° C. that is over an Ar3 critical point; and hot-winding the hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet at 40 to 80% cold reduction ratio.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a cold-rolled steel sheet for an exterior plate of an automobile, such as a door, hood, and trunk lid, more particularly a method for manufacturing a cold-rolled steel sheet that is provided with excellent strain aging resistance at room temperature and bake hardenability by adjusting the amounts of niobium (Nb) and aluminum (Al) for fixing carbon and nitrogen of a solid-solution element to a low-carbon steel and appropriately adjusting the amounts of manganese (Mn) and phosphorus (P) to adjust the strength of the steel, while maintaining high yield strength required for an exterior plate and a finished product after painting-heat treatment.

BACKGROUND ART

Recently, cold-rolled steel sheets for vehicles require high strength for improving fuel efficiency by decreasing weight of the vehicles and for decreasing weight of the car body, and also require sufficient yield strength and tensile strength, good property press-forming property, spot weldability, fatigue property, and painting-corrosion proof, etc.

In general, steel sheets have opposite characteristics in terms of strength and formability, but dual phase steel sheets and bake hardenable steel sheets have been known as steel sheets that satisfy both properties.

In addition to having a low press-forming property as compared with high tensile strength above 40 kgf/mm2 grade, manufacturing of dual phase steel sheets are costly due to significant addition of alloy elements, such as manganese and chrome.

On the other hand, bake hardenable steel sheets show yield strength close to soft steel sheets, when the tensile strength is 40 kgf/mm2 grade or less, so that they have excellent ductility and their yield strength increases when bake-hardening after press-forming.

The bake hardening is a process using a kind of strain aging that is generated by fixing electric charges that are generated in deformation of carbon or nitrogen that is interstitial elements dissolved in the steel. As the amount of dissolved carbon and nitrogen increases in the solid solution, the level of bake hardening increases; however, natural aging can be accompanied by excessively dissolved elements and press-forming property may deteriorates. Therefore, it is important to control the amount of dissolved elements.

Steel sheets for exterior plates of vehicles have been manufactured to ensure bake hardenability by appropriately adjusting the amount of titanium (Ti) or niobium (Nb) added in ultra low carbon aluminum killed steel to adjust the amount of dissolved elements in the steel, and to ensure yield strength by adding phosphorous (P), manganese (Mn), and silicon (Si) etc., which are solution strengthening elements.

A method of controlling the amount of remaining carbon by adding titanium to manufacture a bake hardenable steel may result in varying qualities of the material because the amount of carbon may significantly change. This is because titanium can bond with a variety of elements, such as nitrogen (N), sulfur (S), carbon (C) in the steel.

Further, according to other examples of manufacturing bake hardenable steel, a method of controlling the amount of remaining carbon by adding niobium (Nb) requires high temperature annealing, and therefore the resulting material's qualify varies depending upon the conditions of the annealing, and the quality of plating may deteriorate in a hot dip plating process that may follow. In addition, a method of ensuring bake hardenability using dissolved carbon has difficulty in securing a long aging-warranty period because the carbon has a relatively high diffusion speed in the steel.

That is, bake hardened steels produced by maintaining carbon atoms dissolved in the steel are of disadvantageous, while bake hardenability is high, as strain aging resistance at room temperature decreases as carbon atoms has a high diffusion speed at room temperature.

Further, nitrogen is not used for the bake hardenability, because most of is educed in a winding process into AlN in the case of aluminum-deoxidized steel=and is educed into Tin at a high temperature in the case of titanium added steel.,

Further, there is an additional technology for ensuring bake hardenability, which removes dissolved carbon by applying high temperature annealing and removes dissolved nitrogen by adding aluminum, for low carbon steel having carbon content of 0.01% or more. However, high temperature annealing has disadvantages in that quality of the material may vary depending on the control conditions and the dissolved carbon may not be sufficiently removed after the annealing.

In this case, even if the dissolved carbon is removed by adding titanium and niobium, formability is deteriorated and the strain aging resistance at room temperature is not sufficiently ensured by the remaining carbon unless management of NbC and TiC are controlled.

Technical Problem

In order to overcome the problems, it is an object of the invention to provide a cold-rolled steel sheet having excellent dent resistance by adjusting the amount of niobium (Nb) and aluminum (Al) for fixing carbon and nitrogen, appropriately adjusting the amount of manganese (Mn) and phosphorous (P) to adjust strength of the steel, and using low-temperature annealing and low-temperature winding to maintain necessary yield strength for an exterior plate and high yield strength in the final product after painting-heat treatment.

Technical Solution

A method of manufacturing a cold-rolled steel sheet according to the present invention includes: providing an alloy steel comprising, by weight % with reference to the total weight of the alloy steel, carbon (C) 0.0050 or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%, and iron (Fe); adjusting the atomic ratio of Nb/C to 1 or more and the atomic ratio of Al/N to 0.5 to 1.5, homogenizing the alloy steel at a temperature of 1150 to 1300° C., hot-rolling the steel alloy to provide a hot-rolled steel sheet at a final hot-rolling temperature to 890 to 950° C. that is at right over an Ar3 critical point; and hot-winding the hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet at a cold reduction ratio from 40 to 80%.

Further, annealing is performed in a temperature range of 750 to 880° C. after the cold rolling.

Further, it is preferable that the hot-winding is performed in a range of 450 to 650° C. temperature.

A cold-rolled steel sheet formed of a steel according to an embodiment of the present invention comprises: carbon (C) 0.005% or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%, by weight %, and the remainder of iron (Fe) and elements inevitably contained in manufacturing the steel, wherein an atomic ratio of Nb/C is adjusted to 1 or more and an atomic ratio of Al/N is adjusted to 0.5 to 1.5.

Advantageous Effects

The present invention is designed to manufacture a cold-rolled steel sheet having excellent strain aging resistance at room temperature and bake hardenability using carbon and nitrogen as solid-solution elements. Accordingly, the present invention has an advantage of manufacturing a cold-rolled steel sheet having excellent strain aging resistance at room temperature and bake hardenability and using low-temperature annealing and low-temperature winding.

Further, the present invention has an advantage of preventing non-uniform machining and ensuring strain aging resistance at room temperature and a long aging-warranty period, by maximally preventing solid-solution carbon to prevent effect of carbon in bake hardening.

Further, according to embodiments of the present invention, since the manganese content is decreased, workability and spot weldability are improved. Reduction of strength of the steel sheet due to reduction of the manganese content is compensated by the bake hardening by controlling precipitates and solid-solution nitrogen. Therefore, the present invention can be stably used for exterior plates of vehicles.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a comparison of an embodiment according to a method of manufacturing a cold-rolled steel sheet with a comparative example having different elements, and showing change of bake hardening value to hot winding temperature.

FIG. 2 is a graph showing changes in bake hardening values over annealing temperature of the embodiment of the present invention and a comparative example.

Preferred embodiments of the invention are described hereafter in detail with reference to the accompanying drawings.

A cold-rolled steel sheet and a method for manufacturing the same according an embodiment of the present invention includes: providing a steel alloy comprising: by weight %, nitrogen (N) 0.02 to 0.05%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.30 or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%, iron (Fe) and inevitable impurities; adjusting the atomic ratio of Nb/C to 1 or less and the atomic ratio of Al/N to 0.5 to 1.5; homogenizing the steel alloy at a temperature ranging from 1150 to 1300° C. that is an austenite region, forming a hot-rolled steel sheet by rolling the steel at a temperature ranging from 890 to 950° C. that is over an Ar3 critical point in the final hot rolling; hot-winding the hot-rolled steel sheet at a temperature range of 450 to 650° C., rolling the hot-rolled steel sheet at a cold reduction ratio ranging from 40 to 80% cold reduction ratio, and performing annealing at a temperature ranging from 750 to 880° C.

Thereafter, a hot dip plated steel sheet can be manufactured by performing hot dip plating at a temperature of 460° C. in a process of galvalume or zinc plating on an alloying hot dip plating line and performing alloying at a temperature ranging from 460 to 560° C.

It is preferable to perform overaging at a temperature of 400° C. after the annealing, but may be skipped when annealing is performed at a low temperature.

The plating temperature of 460° C. is a temperature in a melter known in the art although it is not limited thereto.

The temperature of hot-winding is performed at a temperature lower than 450° C., nitrogen can be bonded into AlN in a slab re-heating process, such that bake hardenability may not be ensured by the nitrogen.

On the contrary, when the winding temperature is higher than 650° C., the bake hardenability can rapidly decrease, and therefore the temperature of the hot-winding may be limited to a range from 450 to 650°.

Further, according to the alloy composition of the present invention, an ultra low carbon steel containing carbon of 0.005 wt % or less can be used as raw steel to minimize the solid-solution carbon, which controls bake hardening of the steel, with the solid-solution nitrogen rather than carbon.

Control using the nitrogen is more advantageous to achieve bake hardening than using the carbon. This is because the nitrogen has a lower diffusion speed in steel than the carbon, such that it is advantageous in strain aging resistance at room temperature. Here, the term “strain aging resistance at room temperature” regards the changes in the quality of steel as time passes, and bake-hardened steel should be ensured in the strain aging resistance at room temperature because it will be used in the automotive manufacturing while after the steel manufacturing.

A very small amount of remaining solid-solution carbon can be maximally removed by adjusting the atom ratio of Nb/C. The atom ratio of Nb/C is adjusted to be 1 or greater, which educes all of the solid-solution carbon in the steel into a precipitate of NbC such that only solid-solution nitrogen is in present in the steel. Accordingly, the solid-solution carbon plays little role in bake hardening.

The solid-solution nitrogen is controlled by aluminum that forms a precipitate with nitrogen. When the solid-solution nitrogen is not appropriately controlled, the strain aging resistance at room temperature and the formability may be deteriorated. The atomic ratio of Al/N for controlling the solid-solution nitrogen can be adjusted to a range from 0.5 to 1.5. This is because when the atomic ratio of Al/N is below 0.5, the strain aging resistance at room temperature may not be stably ensured, whereas when it is over 1.5, appropriate amount of solid-solution nitrogen may not be ensured, such that the bake hardenability can deteriorate.

Further, the alloy composition of the invention improves workability and spot weldability by reducing the content amount of manganese that can deteriorate the workability and spot weldability. Reduction of strength of the cold-rolled steel sheet that is caused by reducing the manganese content can be compensated by homogenizing and making the structure minute by NbC and AlN precipitation hardening.

Components contained in the cold-rolled steel sheet of embodiments of the present invention are as follows, in reference to weight % (referred to as % hereafter).

1. Carbon (C) 0 005% or less

When the amount of carbon is 0.005% or higher, the amount of niobium Nb for fixing the carbon can increase, such that not only manufacturing cost of the steel may increase, but workability of the steel may decrease.

Further, when fixing the carbon using the niobium is insufficient, aging may rapidly progress due to the carbon, such that the strain aging resistance at room temperature of the steel may be reduced. Therefore, the amount of the carbon is limited to 0.005% or less.

2. Silicon (Si): 0.3% or less

The silicon (Si) can increase activity of the carbon present in a solid solution state in the steel, such that the strain aging resistance at room temperature can deteriorate and the quality of plating can be significantly reduced. Further, although as the amount increases, the strength may be increased by solid solution hardening, but which decreases ductility, such that the maximum added amount of silicon is limited to 0.3%.

3. Manganese (Mn): 0.1 to 1.0%

The manganese (Mn) is in present in the solid-solution state in the steel and has a function of increasing the strength of the steel. However, the amount of 1.0% or more can largely decrease the ductility, such that the maximum added amount of manganese may be limited to 1.0%. On the other hand, when no manganese is added to the steel, hot shortness may be caused by sulfur present in the steel, such that the minimum added amount of manganese is preferably limited to 0.1%.

4. Phosphorous (P): 0.005 to 0.1%

The phosphorous is present in the solid-solution state in the steel has a function of increasing the strength of the steel. The amount of 0.1% or more may considerably decrease the ductility and weldability of the steel, such that the maximum added amount of the phosphorous may be limited to 0.1%. However, when no manganese is added to the steel, it may be difficult to ensure sufficient strength of the steel, such that the minimum added amount of the phosphorous is preferably limited to 0.005%.

5. Niobium (Nb): 0.015 to 0.04%

The niobium is added to fix the carbon present in the solid-solution state in the steel. The solid-solution carbon present in the steel prevents a cold-rolling collective structure from being formed, such that the workability of the steel deteriorates. Further, when carbon in the solid-solution state exists, the strain aging resistance at room temperature is deteriorated by rapid diffusion of the carbon, such that a sufficient amount of niobium is needed to fix the solid-solution carbon. The necessary amount of niobium is set such that the atom ratio of Nb/C is 1 or more; therefore, the minimum amount is limited to 0.015% and the maximum is limited to 0.04% in consideration of the amount of carbon.

6. Nitrogen (N): 0.002 to 0.005%

In general, the nitrogen (N) is an element that is inevitably added to the steel; however, it is needed to adjust the added amount of nitrogen in the present invention because the present invention controls the bake hardenability using the nitrogen. When the added amount is too small, it is difficult to ensure the bake hardenability and when the added amount is too large, it may be possible to ensure sufficient bake hardenability by the nitrogen, but may cause aging due to the solid-solution nitrogen and deteriorate the workability. Therefore, the added amount of nitrogen is in the range of 0.02 to 0.005%.

7. Aluminum (Al): 0.001 to 0.03%

The aluminum is also added to deoxidize the steel, but is used to control the bake hardenability by bonding with the nitrogen in the present invention. When the amount of aluminum is 0.001% or less, deoxidization is decreased and oxygen is in present in the steel. Accordingly, when elements that form oxidized substances, such as manganese and silicon, are added during manufacturing of the steel, manganese oxide and silicon oxide are formed, such that element control of the silicon etc. is difficult. However, when the amount of aluminum is 0.03% or more, unnecessarily excessive amount is added, such that it reacts with nitrogen being in present in the steel and forms an aluminum nitride precipitate. Therefore, the bake hardenability by the nitrogen cannot be achieved. Accordingly, the maximum added amount of the nitrogen is limited to 0.03%

In addition, the sulfur (S) is an element that is generally inevitably contained during manufacturing of the steel, such that the addition range is limited to 0.02% or less.

The following Table 1 shows an embodiment of the invention and a comparative example each having different components.

TABLE 1 Steel Chemical element No. C Nb Mn P S Al N Al/N Reference 1 0.0023 0.028 0.2 0.011 0.007 0.005 0.0024 1.11 Embodiment 2 0.0030 0.030 0.4 0.040 0.005 0.005 0.0030 0.86 Embodiment 3 0.0021 0.025 0.6 0.030 0.005 0.006 0.0035 0.89 Embodiment 4 0.0031 0.030 0.3 0.060 0.005 0.010 0.0044 1.18 Embodiment 5 0.0022 0.020 0.2 0.020 0.005 0.040 0.0025 8.30 Comparative example 6 0.0025 0.050 0.2 0.011 0.006 0.02 0.0024 4.30 Comparative example 7 0.0023 0 0.2 0.011 0.006 0.01 0.0048 1.08 Comparative example 8 0.0025 0.018 0.3 0.06 0.005 0.045 0.0034 6.8 Comparative example

In Table 1, the Embodiments and Comparative examples are achieved by maintaining the ingot of the solid-solution steel for two hours in a heating furnace of 1250° C. and then hot-rolling it, in which the final temperature of the hot rolling is 900° C., the temperature of hot-winding is 560° C., and cold rolling is performed at 70% cold reduction ratio.

The cold-rolled sample is cooled at a cooling speed of −3° C./sec and continuously annealed at a temperature of 800° C., and the sample after the continuous annealing has undergone a tensile test in a universal testing machine.

The following Table 2 shows changes in the mechanical properties according to heat treatment conditions and manufacturing conditions of the embodiment and comparative example of Table 1.

TABLE 2 Mechanical property Yield Tensile Elongation Steel strength strength ratio BH AI No. (MPa) (MPa) (%) (MPa) (MPa) Reference 1 182 283 46 35 23 Embodiment 2 230 355 39 37 24 Embodiment 3 233 357 40 33 22 Embodiment 4 240 360 38 40 20 Embodiment 5 170 280 45 20 10 Comparative example 6 160 280 47 0 0 Comparative example 7 210 270 45 48 38 Comparative example 8 230 350 38 25 22 Comparative example

As shown in Table 2, sample Nos. 1 to 4 correspond to embodiments of the present invention and have tensile strength of 270 to 360 MPa, elongation ratio of 38 to 47%, bake hardening strength of 33 to 40 MPa, and aging index of 30 or less, such that they achieve high-strength steel, maintain excellent ductility, have high bake hardenability and excellent strain aging resistance at room temperature.

On the other hand, in comparative examples Nos. 5, 6, and 8, the addition amount of Al is high, such that even though the winding process is performed at a low winding temperature, sufficient bake hardenability cannot be ensured by the aluminum fixing the nitrogen.

Further, in comparative No. 7, niobium is not added, such that a large amount of carbon is in present in the solid-solution state in the steel, and accordingly, the bake hardenability is high, but the strain aging resistance at room temperature is low.

FIG. 1 is a graph showing changes in bake hardening values according to hot-winding temperature in one example of each of the comparative example and the embodiment (Embodiment No. 1 and Comparative Example No. 5) and FIG. 2 is a graph showing changes in bake hardening values according to annealing temperature.

It can be seen from FIG. 1 that as the winding temperature of Embodiment No. 1 decreases, the bake hardenability increases, and particularly, the bake hardenability rapidly increases under 600° C.

This is because precipitation of AlN is delayed when the hot-rolling winding temperature decreases, such that nitrogen in a large amount of solid-solution state can exist.

In Embodiment Nos. 1, 2, 3, and 4 having sufficient solid-solution nitrogen is ensured in the hot-rolling winding process, as shown in FIG. 2, it is possible to ensure sufficient bake hardenability even at a low annealing temperature, such that low-temperature annealing is possible. The lower the annealing temperature, the more the energy is saved and the alloying dip plating property is improved. 

1. A method of manufacturing a cold-rolled steel sheet, comprising: adding to iron (Fe), by weight %, carbon (C) 0.005% or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1.0%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03% to provide a steel; adjusting the atomic ratio of Nb/C to 1 or more and the atomic ratio of Al/N to 0.5 to 1.5, homogenizing the steel at a temperature of 1150 to 1300° C., hot-rolling the steel at a final hot-rolling temperature to 890 to 950° C. that is over an Ar3 critical point; and hot-winding the hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet at 40 to 80% cold reduction ratio.
 2. The method of manufacturing a cold-rolled steel sheet according to claim 1, wherein annealing is performed in a range of 750 to 880° C. after the cold-rolling.
 3. The method of manufacturing a cold-rolled steel sheet according to claim 1, wherein the hot-winding is performed in a temperature range of 450 to 650° C.
 4. A cold-rolled steel sheet formed of a steel comprising: carbon 0.005% or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%, by weight %, and the remainder of iron (Fe) and elements inevitably contained in manufacturing the steel, wherein an atomic ratio of Nb/C is adjusted to 1 or more and an atomic ratio of Al/N is adjusted to 0.5 to 1.5.
 5. A method of manufacturing a steel sheet, the method comprising: providing a steel alloy comprising iron (Fe), carbon (C), nitrogen (N), manganese (Mn), phosphorous (P), niobium (Nb), silicon (Si), sulfur (S), aluminum (Al), wherein the atomic ratio of Nb/C is 1 or greater, and the atomic ratio of Al/N is from 0.5 to 1.5 in the steel alloy, hot-rolling the steel alloy to provide a hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet.
 6. The method of claim 5, wherein providing the steel alloy comprises: providing a raw steel material; and adding to the raw steel material one or more elements selected from the group consisting of carbon (C), nitrogen (N), manganese (Mn), phosphorous (P), niobium (Nb), silicon (Si), sulfur (S), aluminum (Al).
 7. The method of claim 6, wherein Nb is added in such an amount to make the atomic ratio of Nb/C1 or greater, wherein C is not added.
 8. The method of claim 6, wherein Nb and C are added in such amounts to make the atomic ratio of Nb/C1 or greater.
 9. The method of claim 6, wherein Al and N are added in such amounts to make the atomic ratio of Al/N fall in a range from 0.5 to 1.5.
 10. The method of claim 6, wherein C is added in an amount of 0.005 wt. % or less with reference to the total weight of the steel alloy, wherein N is added in an amount of 0.002 to 0.005 wt. % with reference to the total weight, wherein Mn is added in an amount of 0.1 to 1.0 wt. % with reference to the total weight, wherein P is added in an amount of 0.005 to 0.1 wt. % with reference to the total weight, wherein Nb is added in an amount of 0.015 to 0.04 wt. % with reference to the total weight, wherein Si added in an amount of 0.3 wt. % or less with reference to the total weight, wherein S is added in an amount of 0.02 wt. % or less with reference to the total weight, wherein Al is added in an amount of 0.001 to 0.03 wt. % with reference to the total weight.
 11. The method of claim 5, wherein providing the steel alloy further comprises: homogenizing the element-added steel material at a temperature between 1150 and 1300° C.
 12. The method of claim 5, wherein a final temperature of the hot-rolling is in a range from 890 to 950° C. over an Ar3 critical point.
 13. The method of claim 5, wherein a cold reduction ratio of the cold rolling is in a range from 40 to 80%.
 14. A cold-rolled steel sheet manufactured from the method of claim
 6. 15. A cold-rolled steel sheet manufactured from the method of claim
 10. 